@Annath: If coasting allows you to maintain your speed without pressing the accelerator, while engine braking causes you to loose momentum going down the hill and need to apply the accelerator again, it's much more of a toss-up which conserves fuel better. The best way to conserve fuel is of course to cut the engine completely off and coast in neutral, and let the transmission restart it (so you don't expend power on the stater motor) when you need the engine again.
–
R..Feb 10 '13 at 17:00

with modern engines leaving the car in gear will require no fuel for going down hill, however, shifting it to neutral will require fuel to keep the engine going so its not as efficient as you may think
–
MauroFeb 11 '13 at 19:56

9 Answers
9

I don't think it will make a difference on gas mileage either way. Anytime the engine is running the front pump on the transmission is being turned by the torque converter and is lubricating the transmission which is also circulating fluid through the transmission cooler.

I don't see how it could cause premature failure of the transmission. Technically you are shifting out of gear and back into gear one extra time when you decide to do this, but it's not really any different that a regular shift so it shouldn't cause extra wear.

"I don't think it will make a difference on gas mileage" -- This is a sideline to the main question, but why is that? When the car is in neutral, I can go longer without having to use the gas pedal. To me, that seems like it ought to improve mileage.
–
Michael MyersMay 11 '11 at 15:21

@Bob: Your comment makes no sense. Engine braking is more efficient than not-braking when the goal is to keep moving, rather than to slow down?? I think you misunderstood what OP is trying to achieve, which is keeping enough momentum while driving downhill so as to maintain speed without pressing the accelerator. By the way, you can save A LOT more gas doing this if you cut the engine and just release the clutch in gear to restart it (no need for starter motor) on a manual, but that's potentially unsafe.
–
R..Aug 7 '11 at 7:59

By the way, I see how engine braking is better when the hill is steep enough to maintain speed (you can keep the engine turning with no fuel), but not on borderline hills where you'll quickly lose speed if you don't either go into neutral of hit the accelerator.
–
R..Aug 7 '11 at 8:04

1

Sure, in that edge case you can save fuel if you shift to neutral and turn the engine off. Whether the fuel saved outweighs the wear and tear from the engine stopping and starting is probably not a bad question to ask at that point.
–
NickFeb 10 '13 at 22:41

Auto transmissions are designed to operate similarly to manual transmissions.
If you are coasting with an automatic transmission, all you are doing is the same as when you coast with a manual transmission.

It cannot damage the box if the engine is running and your transmission pump is working.
But the downside is: do not try to put it into D when in high speed, because that can burn your clutches gradually by making them slip when they are engaging at high speed.
And that will make a hole in your pocket.

Theoretically it cannot damage the box, but is better to follow the guidelines in your vehicle's manual for your own safety.

It all depends on the vehicle in question as to whether it saves fuel, and/or is detrimental to the auto tranny.

As long as the engine is running, it is rotating the torque converter, which in turn is pumping fluid around the auto box. This keeps all the bands, clutches, servos etc lubricated no worries.
When going back into gear, if you're not rolling up to a stop sign or red light or similar, it may help to increase the engine revs a little to match the revs the vehicle would be doing at that road speed. This will reduce any shock loading on the internals.

As for saving fuel, it depends on the vehicle, carbie cars, and early injected vehicles, it will definitely save fuel.
On later vehicles, with more sophisticated electronics, Your mileage may vary. Some vehicles will cut fuel to all/most injectors in a closed throttle position. Where as at idle, and in neutral, all cylinders are getting fuel to keep the engine running smoothly.

It is fine to coast in an automatic in neutral as long as the engine is running. If you stop the engine completely and coast (as is often the case when being towed), you can damage the transmission.

The reason is that the transmission circulates fluid for lubrication and cooling, and this circulation is powered by the engine. Rotating the drive wheels (coasting or being towed) without the engine running causes transmission parts to be moving without sufficient lubrication or cooling, which will wear them out much more rapidly.

I live in Colorado, long stretches of downhill driving, I always shift into neutral and I can see that my rpms drop so i should be conserving fuel. I never shift back into drive until I come to a stop sign or slow down enough to do it safely. Don't shift back into gear at high speeds. Ive sometime gone 15-20 miles in neutral down some Rocky Mountain passes and have to brake now & then to keep from going tooo fast.

You do realize driving your car in neutral is probably illegal in Colorado, as I know it is in Montana (another Rocky Mountain state with even more mountains than Colorado). Secondly, when you leave your vehicle in gear, with the throttle closed you will get better gas mileage than by putting it in neutral because of the way modern fuel injection works. When the throttle is closed and the vehicle is decelerating, fuel output at the injector goes to zero. When idling, which it would be when in neutral, you'd be using more fuel ... thanks to @BobCross for pointing this out to me.
–
Paulster2Jul 11 '14 at 10:41

I frequently coast in neutral, w/ a noticeable boost to mileage. I'm no mechanic, but I like to think I have a mechanical sympathy, and I'm careful when re-engaging 'D' to use some combination of moderating speed (waiting for levelling out or uphill) and touching the accelerator, so that the transition feels smooth. I only do this on interstate, and certainly not where I need to slow or stop at the bottom of a hill. Haven't noticed any transmission problems yet.

Most manuals for cars automatic transmissions state that the car should not be towed with all four wheels on the ground for more than 50 miles. I know of several cars that have had to have their gearboxes/torque converters replaced because they were towed too far. Coasting in "neutral" is going to cause the same issue as far as I am aware.
–
MauroAug 9 '11 at 12:35

1

Wow, show me long shallow grade that is over 50 miles long. I would love to coast down on that.
–
theUgMay 28 '12 at 22:06

4

@Mauro The difference in towing and coasting is the engine is still running and that is turning the front pump in the transmission keeping the transmission lubricated. When you tow the vehicle the transmission isn't being lubricated.
–
Move more comments link to top♦Feb 10 '13 at 18:02

Yeah, they are separate situations. Coasting in neutral is not going to hurt anything.
–
NickFeb 10 '13 at 22:38

Why do you think tow trucks hook up cars facing backwards?
It's so the 2 wheels on the ground are the front ones - If you tow a car with the back wheels on the ground, it can ruin the car/gearbox.
In theory you cannot tow a full time 4wd with any wheels on the ground!

My shogun has an option of 2wd, so if I was going to tow it, I'd select 2wd then hook it up to the tow truck with the front wheels on the ground.

My Avalon gets around 15-17 mpg in short-trip suburban driving, and around 30 mpg when kept at 60 mph or so over long stretches on interstates along the East Coast. On a recent trip largely through NY State with some miles-long downhill coasting on state highways it got well over 30 mpg -- in fact, even with the occasional pit stop or slow-down-speed-up due to exurban town travel, it was making 33 mpg or more. Even around mid-NJ I can start a trip averaging 30 mph or so on two-lane blacktop with the mileage meter reading "15 mpg since last refill" and end up 12 miles later with the reading at 20 mpg or more. There may be some hidden problem (damage?) with the automatic transmission, but it's not yet evident after several thousand miles of occasional coasting.

Welcome to the site. In case it's not clear, this is not an open-ended discussion site. We're looking for answers to the questions posed. While your anecdote is somewhat related to the topic, it does not actually answer the original question.
–
Bob Cross♦Nov 1 '13 at 11:44

Sorry for being unclear. I have few opportunities to coast in neutral around town, but do so when going downhill on longer trips -- which yields a 10% or better increase in MPG.
–
Butch BenjaminNov 2 '13 at 22:33